With recent advancement of digital technologies, electronic equipment such as portable information devices and information home appliances has been developed to provide higher functionality. As the electronic equipment has been developed to provide higher functionality, development of miniaturized and higher-speed semiconductor elements incorporated into the electronic equipment is progressing at a high pace. Among them, the use of a large-capacity nonvolatile memory which is represented by a flash memory has been spreading at a rapid pace. Furthermore, as a next-generation new nonvolatile memory which has a potential to replace the flash memory, a resistance variable nonvolatile memory device including a so-called resistance variable element has been researched and developed. The resistance variable element is defined as an element which has a characteristic in which its resistance values change reversibly in response to electric signals and is able to store information corresponding to the resistance values in a nonvolatile manner.
As an example of a large-capacity nonvolatile memory element incorporating the resistance variable element, a cross-point nonvolatile memory has been proposed. An element is disclosed, which is configured to include a resistance variable layer as a memory section and a diode element as a switching element (see, for example, Patent Literature 1).
FIGS. 14A and 14B show a nonvolatile memory device 50 including a conventional resistance variable element. FIG. 14A is a perspective view of a cross-point memory cell array including bit lines and word lines and memory cells provided at cross-points of the bit lines and the word lines, respectively, while FIG. 14B is a cross-sectional view of a memory cell 280, a bit line 210 and a word line 220 which are taken along a bit line direction. A resistance variable element 260 has a structure in which a resistance variable layer 230 adapted to change its electric resistance by electric stresses to thereby store data is sandwiched between an upper electrode 240 and a lower electrode 250. A two-terminal non-linear element 270 having a non-linear current-voltage characteristic in which the non-linear element 270 is capable of flowing a current bidirectionally, is provided at the upper side of the resistance variable element 260. The memory cell 280 is constituted by a series circuit including the resistance variable element 260 and the non-linear element 270. The non-linear element 270 is a two-terminal element having a non-linear current-voltage characteristic in which a current changes at an inconstant rate with respect to a voltage change, like a diode or the like. The bit line 210 which serves as an upper wire is electrically connected to the non-linear element 270. The word line 220 which serves as a lower wire is electrically connected to the lower electrode 250 of the resistance variable element 260. As the non-linear element 270, for example, a varistor (ZnO, SrTiO3, etc.) having a current-voltage characteristic which is symmetric bidirectionally and is non-linear, is used, because it flows a current bidirectionally when data is rewritten in the memory cell 280. It is recited that with the above configuration, it becomes possible to flow a current with a current density required to rewrite data in the resistance variable element 260, which is not less than 30 kA/cm2, thereby attaining a larger capacity.